A Versatile Droplet Sizing Spectrometer for Aerosol Hygroscopic Growth Measurements from Research Aircraft

There are currently no viable techniques for airborne observations of the size-resolved hygroscopic growth properties of ambient aerosols. The change in size of ambient particles with relative humidity significantly impacts the global atmosphericradiation budget, regional visibility and wet deposition of pollutants. This work focuses on the development of a new method to measure size-resolved water uptake by aerosols that employs differential mobility analysis in-concert with multiplehumidification and optical sizing steps. The particle size range and relative humidity limitations of current techniques are addressed by allowing a much broader size range and multiple humidities to be explored over timescales relevant for aircraftsampling. The new instrument system will be designed to operate in a fully automated fashion for field-deployment on the Navy CIRPAS Twin Otter. The measurements will allow local vertical profiles to be made to investigate the role of particle-bound waterin determining column radiative properties and for intercomparisons with satellite-derived optical depths. Overall, the new technique will provide results relevant to improving understanding of the impacts of ambient particle hygroscopic growth onradiative transfer, cloud droplet formation, and to validate new aerosol thermodynamic models. The results from the proposed measurements will help validate satellite retrievals of aerosol optical properties. Given the complex relationship that has beenobserved between ambient particle size, composition and hygroscopicity, direct measurements of water uptake as proposed here are needed. Humankind's activities may be perturbing cloud properties through changes in the ambient aerosol that serve as CCN.Results from the proposed measurements would provide direct input to models attempting to simulate cloud droplet growth and activation for a variety of ambient aerosols. Furthermore, many of the problems associated with existing hygroscopic growthmeasurement techniques are alleviated by design innovations in the new system proposed here. Numerous research groups in the US and Europe are potential purchasers of the new water uptake measurement technology. NSF, DOE and NOAA operate research aircraft with state-of-the-art aerosol instrumentation and would be potential customers. Futureversions of the instrument could be miniaturized for autonomous operation on small remotely piloted aircraft and drones. The smaller device could find extensive use in DOE Atmospheric Radiation Measurement (ARM) program field studies and other remoteapplications. Although the new technology will be geared toward aircraft measurements, the system will also be capable of ground-based operation. As the EPA promulgates new standards for ambient particulate matter loading, new techniques for understanding theproperties of ambient aerosols will be required. The new EPA standards are being driven primarily by the health effects of ambient particles, although visibility reduction is another key issue. The proposed autonomous water-uptake measurement technologyis pertinent to both health and visibility impacts of ambient aerosols, and there may be opportunities for large commercial markets that include municipalities and regional air quality monitoring districts. Future, lower-cost versions of the technologycould be optimized for ground-based monitoring applications.In the wake of September 11th, the need for new technologies to address bio-warfare aerosols has increased. The new water-uptake measurement system has numerous potential applications including laboratory studies to understand how biological agentsrespond to increased RH so that atmospheric transport and deposition of these compounds within human airways can be properly modeled under various release scenarios. The current water uptake system design could be adapted for application in the area ofbio-aerosol detection by comparing simultaneous ambient number size distribution and hygroscopic growth measurement results to known hygroscopic response and size characteristics of bio-agents studied in the laboratory. Pharmaceutical companies could alsouse the technology to gauge the human-airway deposition efficiency and deep-lung dose delivery effectiveness of an aerosolized drug-delivery system. Although the system proposed here does not have integrated chemical composition measurement capabilities,BMI is working on a new technique for on-line composition measurements that could be integrated with the proposed water-uptake system in the near future.